Subwavelength-diameter silica wires for low-loss optical waveguiding

ABSTRACT

The present invention provides nanometer-sized diameter silica fibers that exhibit high diameter uniformity and surface smoothness. The silica fibers can have diameters in a range of a about 20 nm to about 1000 nm. An exemplary method according to one embodiment of the invention for generating such fibers utilizes a two-step process in which in an initial step a micrometer sized diameter silica preform fiber is generated, and in a second step, the silica preform is drawn while coupled to a support element to form a nanometer sized diameter silica fiber. The portion of the support element to which the preform is coupled is maintained at a temperature suitable for drawing the nansized fiber, and is preferably controlled to exhibit a temporally stable temperature profile.

RELATED APPLICATION

This application claims priority to a provisional application entitled“Subwavelength-diameter silica wires for low-loss optical waveguiding”having a Ser. No. 60/530,467 and filed on Dec. 16, 2003.

FEDERALLY SPONSORED RESEARCH

The invention was made with Government support under contractPHY-0117795 awarded by National Science Foundation (NSF). The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to silica wires havingsub-micrometer or nanometer-sized diameters, and methods for formingsuch wires, and more particularly, to fabrication methods that providenano-sized silica wires having enhanced uniformity and surfacesmoothness.

Miniature silica waveguides with diameters or widths larger than thewavelength of transmitted light are widely utilized in opticalcommunications, sensors, and other applications. Silica wires withdiameters more than one micrometer, which allow multimode waveguiding ofvisible and infrared radiation, can be made by drawing high-purity glassfibers from a laser-heated melt. Although photonic applications canbenefit from incorporation of silica submicrometer- ornanometer-diameter wires (SMNW), the fabrication of such silica wirespresents a number of challenging problems. In particular, boththeoretical and experimental results indicate that the laser powerrequired for drawing silica SMNWs with a uniform diameter from alaser-heated melt is impractically large. Further, it is difficult todraw SMNWs having uniform diameters from a flame-heated melt becauseturbulence and convention render controlling the temperature gradient inthe drawing region problematical. Moreover, silica nanowires obtainedrecently by employing other methods exhibit large diameter fluctuationsand sidewall roughness that render these wires unsuitable for low-lossoptical waveguiding.

Hence, there is a need for silica fibers having submicrometer- ornanometer diameters that exhibit enhanced diameter uniformity andsurface smoothness.

There is also a need for methods that allow fabrication of such silicafibers.

SUMMARY OF THE INVENTION

Silica fibers having average cross-sectional diameters in a range ofabout 20 nm to about 1000 nm, methods for their fabrication as well asmicrophotonic devices incorporating them, are disclosed. The silicafibers, which exhibit a high degree of diameter uniformity, can beassembled over a substrate having an index of refraction less than about1.4, and preferably less than about 1.1, for incorporation in a varietyof microphotonic devices.

In one aspect, the present invention provides a method of fabricating anano-sized optical fiber that includes thermally coupling a silica(SiO₂) preform, shaped as a fiber having an average diameter in a rangeof about 1 to about 20 micrometers, to a support element. The preform ismaintained at a temperature below its melting point but high enough topermit its deformation, e.g., a plastic deformation, in response to atensile force applied thereto. A nano-sized fiber, having an averagediameter in a range of about 50 nm to about 1000 nm, is then drawn fromthe preform.

In a related aspect, a selected portion of the support element is heatedto maintain the portion of the support element thermally coupled to thepreform, and consequently the preform, at a desired temperature.Preferably, a substantially steady temperature profile, i.e., atemperature profile that is temporally stable at least over the timeperiod required for drawing the nanosized fiber, is established acrossthe portion of the support element that is coupled to the preform.Further, although the temperature across this portion of the preform ispreferably substantially uniform, it can exhibit a temperature gradient,albeit preferably a temporally stable gradient, with a maximumtemperature variation in a range of about 50 to about 200 C.

A variety of materials, such as, sapphire, diamond, and Y₂O₃—ZrO₂, canbe employed for forming the support element. In general, the supportelement preferably exhibits a melting temperature higher than themelting temperature of the silica wire, e.g., a melting temperaturehigher than about 1800 C. In addition, the support element preferablyhas a heat capacity that is sufficiently high to substantially inhibittransfer of temperature fluctuations in the heated portion of thesupport element to the portion to which the preform is coupled, at leastduring the time required for drawing the nanosized fiber. For example,the support element can have a heat capacity in a range of about 0.5 toabout 2.5 J/gK.

In another aspect, the size of the portion of the support elementcoupled to the preform and its distance from the heated portion of thesupport element, together with judicious selection of the materialforming the support element, allow the portion coupled to the preform tofunction as a thermal reservoir element that ensures a steadytemperature distribution across the drawing region by reducing the rateof transfer of temporal fluctuations in the temperature of the heatedportion to the drawing region.

In one aspect, the support element can be a tapered sapphire rodextending from a base portion to a tip portion to which the silicapreform can be coupled. For example, the silica preform can be wrappedaround the tip portion while the tip portion is heated, for example, byplacing a selected segment of the sapphire rod in a flame. The tipportion is preferably heated to a temperature that is sufficiently highto cause softening of the preform, thereby facilitating its coupling tothe support element. Subsequently, the preform can be drawn to generatethe nano-sized silica fiber.

A variety of heating sources can be employed for maintaining thetemperature of the drawing region at a desired value. For example,flames of CH₃OH or C₂H₅OH can be employed. Alternatively, electricalheating elements can be utilized for this purpose.

In other aspects, the invention provides an optical fiber that includesa silica wire having an average diameter in a range of about 20 nm toabout 1000 nm, or in range of about 20 nm to about 500 nm, or in a rangeof about 20 nm to about 100 nm, or in a range of about 20 nm to about 50nm. Such a silica wire is also herein referred to as silica nanowire, anano-sized silica wire or a silica submicrometer- or nanometer-diameterwire (SMNW). A ratio of maximum variation of the fiber's diameter over alength of the silica fiber (ΔD/L) can be less than about 10⁻³, orpreferably less than 2×10⁻⁵, or more preferably less than about 2×10⁻⁶.Further, the silica wire can exhibit a smooth sidewall with aroot-mean-square roughness less than about 0.5 nm, and more preferablyless than about 0.2 nm. Moreover, the silica wire can have a length in arange of about 1 micron to about 100 millimeters.

The silica nanowire can be doped with a selected dopant, such as, Er⁺³,Nd⁺³ or Yb⁺³ ions, or any other suitable dopant. Such doped silicananowires can be employed in a variety of microphotonic applications, asdiscussed in more detail below. Moreover, the silica optical fiber canexhibit a tensile strength in a range of about 2.5 to about 5.5 GPa.

In other aspects, the invention provides a microphotonic device having asubstrate with an index of refraction less than about 1.4, and one moresilica nanowires having an average diameter in a range of about 20 nm toabout 1000 nm (or in range of about 20 nm to about 500 nm, or in a rangeof about 20 to about 100 nm, or in a range of about 20 nm to about 50mm) that are assembled over the substrate. The index of refraction ofthe substrate can be, for example, in a range of about 1 to about 1.4,or in a range of about 1 to about 1.3, or in a range of about 1 to about1.2, or in a range of about 1 to about 1.1, or in range of about 1 toabout 1.05. For example, the substrate can be formed of silica aerogelhaving an index of refraction of about 1.03.

In a related aspect, a microphotonic branch coupler is disclosed havinga substrate with an index of refraction less than about 1.4, e.g., in arange of about 1 to about 1.4 or in a range of about 1 to about 1.3 orin a range of about 1 to about 1.2 or in a range of about 1 to about1.1, and two silica nanowires having average diameters in a range ofabout 20 nm to about 1000 nm that are mounted on the substrate. Thesubstrate can be, for example, a silica aerogel substrate. The silicananowires are assembled on the substrate so as to have longitudinal(length-wise) contact over a selected distance such that a fraction ofradiation originally guided by one fiber is coupled to the other fiberwhile the remaining radiation continues its propagation in the originalradiation-guiding fiber. The coupling distance between the two fiberscan be in a range of about 1 micrometer to about 100 micrometers, andtypically in a range of about 2 micrometers to about 4 micrometers.

In other aspects, the invention provides a microphotonic coupler thatincludes a substrate (e.g., silica aerogel) having an index ofrefraction less than about 1.4 (e.g., in a range of about 1 to 1.1) andtwo silica fibers that are assembled on the substrate. Each fiber has aproximal end and a distal end and an average diameter in a range ofabout 20 nm to about 1000 nm. A distal portion of one fiber is inlength-wise contact with a proximal portion of the other fiber such thatradiation guided by one fiber can couple to the other at the contact.The contact portion can provide an overlap in a range of about 1 micronto about 100 microns (e.g., in a range of about 2 to about 4 microns)between the two fibers.

In yet another aspect, the invention provides a method of forming anano-sized fiber that includes the steps of thermally coupling aproximal portion of a silica fiber having a diameter in a range of about1 to about 20 microns to a tip portion of a thermally conductive supportelement such that another portion of the fiber extends vertically downfrom the tip portion of the support element. The support element can beheated at a selected distance from the tip portion so as to raise thetemperature of the tip portion (e.g., to a temperature in a range ofabout 1500 C to about 1800 C) so as to soften the fiber portion coupledto the support element. A gravitational pull generated by the verticallyextending fiber portion causes drawing of the fiber into a wire having adiameter in a range of about 20 nm to about 1000 nm.

Further understanding of the invention can be obtained by reference tothe following detailed description in conjunctions with the associateddrawings, described briefly below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a silica fiber formed in accordance withthe teachings of the invention having a cross-sectional diameter in arange of about 50 nm to about 1 micrometer,

FIG. 2 is a flow chart delineating various steps in an exemplary methodaccording to one embodiment of the invention for fabricatingnanometer-sized diameter silica fibers,

FIG. 3A schematically illustrates one exemplary method for fabricatingsilica fiber preforms having micrometer-sized diameters,

FIG. 3B schematically illustrates another method for fabricating silicafiber preforms having micrometer-sized diameters,

FIG. 4A schematically depicts a second step in a fabrication methodaccording to the teachings of the invention for generating silicasubmicrometer- or nanometer wires,

FIG. 4B schematically illustrates drawing a nanosized fiber from apreform in a fabrication method depicted in FIG. 4A,

FIG. 5 schematically depicts a fabrication method in accordance withanother embodiment of the invention in which an electric heater isemployed for heating a support element to which a silica preform iscoupled,

FIG. 6 is an electron micrograph of a nanometer-sized diameter silicawire formed in accordance with the teachings of the invention,

FIG. 7 is a TEM image of a 240-nm diameter silica wire formed inaccordance with the teachings of the invention at a magnification of200,000,

FIG. 8 is a TEM image of the sidewall of a 330-nm diameter silica wireformed in accordance with the teachings of the invention obtained at amagnification of 700,000,

FIG. 9 is an SEM image of a 15-micrometer diameter micro-ring made witha 520-nm diameter silica wire fabricated in accordance with theteachings of the invention,

FIG. 10 is an SEM image of two twisted 330-nm diameter silica wiresfabricated in accordance with the teachings of the invention,

FIG. 11 is an SEM image of a bent 280-nm diameter silica wire with abending radius of 2.7 micrometers,

FIG. 12 schematically illustrates an experimental step-up for couplinglight from a launching nanosized silica wire to a guiding nanosizedsilica wire via evanescent wave coupling,

FIG. 13 is an optical microscope image of a 390-nm diameter tapercoupling light into a 450-nm diameter silica wire,

FIG. 14 is a long-time exposure micrograph of 633-nm wavelength lightguided by a 360-nm diameter silica wire of the invention in air andintercepted by a 3-micrometer diameter guiding wire (on the right of theimage),

FIG. 15 is an optical microscope image of 633-nm wavelength light guidedby a 550-nm diameter silica wire with its left half suspended in air andthe right half placed on a MgF₂ crystal,

FIG. 16 presents graphs depicting experimentally measured optical lossof exemplary nanosized silica fibers fabricated in accordance with theteachings of the invention as a function of fiber length at twodifferent transmission wavelengths,

FIG. 17 is an optical microscope image of 633-nm light traveling througha sharp bend with a radius of 5.6 micrometers in a 510-nm diametersilica wire of the invention,

FIG. 18 schematically depicts a silica nanowire assembled on a silicaaerogel substrate according to the teachings of the invention,

FIG. 19 is an SEM image of a 1.5 micron long silica wire segment cutfrom a wire according to one embodiment of the invention having adiameter of 160 nm, showing flat end faces of the wire segment,

FIG. 20 is an SEM image of a 9 micron diameter loop on a sapphiresubstrate formed by elastic bending of a 200 micron diameter wire,

FIG. 21 is an SEM image of a 5-micron radius plastic bend from in a410-nm silica wire mounted on a sapphire substrate in accordance withone embodiment of the invention,

FIG. 22 is an SEM image of double plastic bends in a 940-nm diametersilica wire formed in accordance with one embodiment of the invention,

FIG. 23 is an SEM image of double plastic bends formed in an 800-nmdiameter silica wire in accordance with one embodiment of the invention,

FIG. 24 is an SEM image of a 450-nm wide silica wire assembled on asilica areogel substrate in accordance with one embodiment of theinvention,

FIG. 25 is an SEM image of an aerogel-supported 530-nm diameter silicawire with a bending radius of 8 microns,

FIG. 26 is an optical microscopy image of a 380-nm diameter silica wireaccording to one embodiment of the invention, guiding radiation with awavelength of 633 nm on a surface of a silica aerogel,

FIG. 27 presents optical loss data at a wavelength of 633 nm forexemplary illustrative straight silica wires supported by a silicaaerogel substrate,

FIG. 28 shows an optical microscopy image of an exemplary 530-nmdiameter silica wire supported on an aerogel silica substrate, guidinglight having a wavelength of 633 nm around a bend with a radius of 8microns,

FIG. 29 presents data corresponding to measured optical loss as afunction of bending radius in a 530 nm diameter silica wire supported ona silica aerogel substrate,

FIG. 30 schematically depicts a microphotonic coupler formed of twosilica wires supported on an aerogel substrate in accordance with oneembodiment of the invention,

FIG. 31 schematically depicts a microphotonic X-coupler formed of twobent silica nanowires assembled on a silica aerogel substrate inaccordance with one embodiment of the invention,

FIG. 32 schematically illustrates two silica nanowires assembled on asilica aerogel substrate that intersect one another at a substantiallynormal angle,

FIG. 33 is an optical microscopy image of two 390-nm diameter silicawires supported by a silica aerogel substrate intersecting one anotherperpendicularly, into one of which light having a wavelength of 633-nmis launched,

FIG. 34 schematically illustrates a silica nanowire according to oneembodiment of the invention doped with a selected dopant,

FIG. 35 is a schematic diagram of a self-modulated taper drawing systemaccording to one embodiment of the invention that is suitable forgenerating nano-sized silica fibers,

FIG. 36 schematically illustrates self-modulation realized in a methodaccording to one embodiment of the invention for drawing a nano-sizedsilica fiber from a larger diameter fiber,

FIG. 37 presents graphs corresponding to measured diameter and diameteruniformity of prototype nano-sized silica fiber as a function of length,

FIG. 38 is an SEM image of a silica bundle formed of three silicananowires,

FIG. 39 shows an SEM image of an 18 micron diameter loop assembled witha 65-nm diameter silica nanowire,

FIG. 40 shows an SEM image of a rope-like twist formed with a 120-nmdiameter silica nanowire on a silicon substrate, and

FIG. 41 presents an SEM image of a 150-nm diameter silica nanowirewrapped around a 4 micron diameter fiber to form a spiral coil.

DETAILED DESCRIPTION

The present invention provides nanometer-sized diameter silica fibers,and methods for their production, that exhibit a high uniformity indiameter size along the fiber's length as well as a highly smoothsurface with a root mean square roughness that can be less than about0.5 mm. As discussed in more detail below, an exemplary method of theinvention for generating such silica fibers utilizes a process in whicha silica fiber preform having a micrometer-sized diameter is thermallycoupled to a portion of a support element. The support element is heatedso as to maintain the fiber at a temperature that allows drawing thepreform in a subsequent step to form a nanometer-sized diameter silicafiber.

FIG. 1 schematically depicts an exemplary silica fiber 10, formed inaccordance with the teachings of the invention, that exhibits across-sectional diameter D with a value in a range of about 50 nm toabout 1100 nm, or in some preferred embodiments in a range of about 20nm to about 1100 nm, and a length L with a value in a range of about 1millimeter to about 100 millimeters. The silica nanowires can also becut into smaller lengths, even down to about 1 micron, as discussed inmore detail below. The ratio of maximum variation of the cross-sectionaldiameter D at different locations along the fiber relative to thefiber's length, i.e., ΔD/L, can be as low as 10⁻³, or 2×10⁻⁵, and morepreferably as low as 2×10⁻⁶. The illustrative silica fiber can alsoexhibit high surface uniformity. For example, the root mean square ofthe roughness of the surface, i.e., root mean square of variations ofsurface topography about an average value, can be less than about 0.5nm, or less than about 0.2 nm.

With reference to a flow chart 12 of FIG. 2, in an exemplary method forgenerating a silica fiber according to one embodiment of the invention,in an initial step 14, a preform silica fiber having a cross-sectionaldiameter in a range of about 1 micrometer to a few tens of micrometers,e.g., 20 micrometers, is generated. A variety of techniques are known inthe art for forming such micrometer-sized silica fibers. For example,with reference to FIG. 3A, in one such method, a portion of a silicafiber 20 having a diameter of a few hundred micrometers, e.g., a glassfiber commercially available from Corning Inc. of N.Y., U.S.A. undertrade designation SMF-28, is heated by a flame after the buffer layer isstripped off, e.g., a flame 22 of CH₃OH, to an elevated temperature,e.g., a temperature in a range of about 1600 C to 1800 C, that causessoftening of the heated portion. The heated fiber is then pulled to forma smaller diameter fiber having a diameter in a range of a fewmicrometers, e.g., 1 to 20 micrometers. Other methods for generating amicrometer-sized diameter fiber are also known. For example, an articleentitled “Carbon dioxide laser fabrication of fused fiber coupled andtaper,” authored by T. Dimmick, G. Kakarantzas, T. Birks, and P. Russel,published in Applied Optics, Nov. 20, 1999, page 6845, and hereinincorporated by reference, describes the use of a CO₂ laser as a heatsource for forming fiber tapers having micrometer-sized diameters.Alternatively, as shown in FIG. 3B, an electric heater 24 can beutilized to heat a starting fiber 26 having a diameter of a few hundredmicrometers to a temperature sufficiently elevated, e.g., glasstransition temperature, that allows drawing a micrometer-sized diameterfiber, e.g., by utilizing a spool 28, from the heated starting fiber.

Referring now to the flow chart 12 as well as FIG. 4A, in step 16, themicrometer-sized silica preform fiber is wound around a tip portion 26a, having an average diameter of about 100 micrometers in thisembodiment, of a heated tapered sapphire rod 26. The winding of thesilica preform fiber can be achieved while the sapphire rod is heated,for example, by a flame 28, by placing one end of themicrometer-diameter silica wire horizontally on the sapphire rod's tip,and rotating the rod about its axis of symmetry to wind the silica wirearound the tip. The winding of the silica preform can be performedwithin the flame or slightly outside of the flame so long as thetemperature at the tip portion is sufficiently high, e.g., in a range ofabout 1500 to 1800 C, to cause at least some softening of the preform,thereby facilitating its winding around the sapphire tip. When thetemperatures within the flame are close or above the melting temperatureof the preform, if the winding is performed with the tip portion withinthe flame, the winding rate is preferably sufficiently fast to inhibitany substantial melting of the preform.

With continued reference to the flow chart 12 as well as FIG. 4B, in asubsequent step 18, the wound preform can be drawn perpendicular to thelongitudinal axis of the sapphire rod in a horizontal plane at a rate ofabout 1 to about 10 mm/sec to form a nanometer-sized diameter silicafiber 30, such as the silica fiber schematically shown in FIG. 1 above.More particularly, in this embodiment, initially, the tip portion, withthe silica preform wrapped around it, is positioned at a selecteddistance, e.g., at a distance of about 0.5 mm, from the edge of theflame. The flame 28 is adjusted until the temperature of the tip portionis below the melting temperature of the silica preform, but sufficientlyelevated to allow plastic deformation of the preform in response to atensile force. For example, the average temperature of tip portion canrange from about 150 C to about 1800 C. In this embodiment, thetemperature of the tip portion is set at about 1600 C, which is slightlybelow the melting temperature of the silica preform, i.e., 1800 C.

In this embodiment, the flame 28 is generated by employing methylalcohol (a CH₃OH torch with a nozzle diameter of about 6 mm). In otherembodiments, other suitable compounds, such as ethyl alcohol (C₂H₅OH),can also be employed. In general, it is preferable to choose a compoundwith a low carbon atom constituency to avoid deposition of carbon fromthe flame onto the preform being drawn.

Although the temperature profile across the heated tip portion of thesapphire rod, i.e., the portion coupled to wound preform, is preferablysubstantially uniform, in many embodiments, a temperature gradient witha maximum temperature variation in a range of about 50 C to about 200 Cis present across the tip portion. This temperature profile or gradientis, however, controlled to remain substantially steady, that is,temporally stable, at least during the time period required for drawingthe nano-sized fiber. In other words, the sapphire tip portion functionsas a thermal reservoir element that responds very slowly, at least overthe time period required for drawing the nano-sized fiber, totemperature fluctuations that may occur in the flame, hence ensuringtemporal stability of the temperature profile across the wound preform.Such temperature stability of the preform during the drawing process inturn helps provide diameter size uniformity and high smoothness of thedrawn nano-sized fiber.

Although a sapphire tapered rod was employed in the above embodiment,other materials can be utilized in other embodiments to form a supportelement to which a silica preform fiber can be coupled. In general,materials suitable for forming the support element preferably exhibit asufficiently high melting temperature, e.g., a melting temperaturehigher than 1800 C, to withstand a rise in temperature sufficient fordrawing the preform (e.g., a refractory material). In addition, the heatcapacity of such materials is preferably sufficiently high such that theportion of the support element coupled to the silica preform can be heldat a steady temperature even in case of rapid fluctuations, i.e.,fluctuations occurring over a time scale shorter than the drawing time,in the heating provided by a heat source, e.g., a flame. For example,the heat capacity of the support element can be in a range of about 0.5to about 2.5 J/gK. Some examples of materials other than sapphiresuitable for forming the support element include diamond and a Y₂O₃—ZrO₂crystal.

A two-step fabrication method according to the above embodiment of theinvention in which a nano-sized diameter fiber is drawn from a preformedmicro-sized silica fiber while maintaining a substantially steadytemperature distribution in the drawing region can be performed byutilizing techniques other than that described above. For example, withreference to FIG. 5, in another embodiment, a micrometer-sized silicapreform is coupled to a support element 32, e.g., a sapphire rod, thatis heated by an electric heating element 34. The heated silica preformcan then be drawn to generate a nano-sized diameter silica fiber.

The technique described in the first embodiment above (FIGS. 4A and 4B)can be employed to obtain silica submicrometer- or nanometer diameterwires (SMNW) with diameters as small as 50 nm and lengths up to tens ofmillimeters. By way of example, FIG. 6 shows a scanning electronmicroscope (SEM) image 36 of 4-mm long silica wire (the wire is coiledup for more facile presentation of its length) according to theteachings of the invention having a diameter of 260 nm.

Further, FIGS. 7 and 8, which are, respectively, high-magnificationtransmission electron microscope (TEM) images of a 240-nm diametersilica wire (at a magnification of about 200,000) and the sidewall 40 aof a 330-nm diameter silica wire 40 (at a magnification of about700,000) formed in accordance with the teachings of the invention, showno visible irregularity in the surfaces of these wires, therebycorroborating high smoothness of these surfaces. In fact, as indicatedabove, the typical sidewall root mean square roughness of prototypewires formed by Applicants in accordance with the above method is lessthan 0.5 nm. The insert of FIG. 8 is an electron diffraction pattern ofthe silica wire, demonstrating that the wire is amorphous. It should beunderstood that the above exemplary experimental data, and the datapresented in the discussions below, regarding silica wires formed inaccordance with the teachings of the invention are presented forillustrative purposes and to show the efficacy of fabrication methods ofthe invention, and are not intended to necessarily indicate optimalsilica fibers that can be obtained by practicing such methods. Inparticular, silica fibers having more uniform diameters and bettersurface smoothness may be fabricated by practicing the methods of theinvention.

The high flexibility and the extended length of silica SMNW of theinvention facilitate their manipulation under an optical microscope toform a variety of topological shapes. By way of example, FIG. 9 shows a520-nm diameter silica wire 42 that is elastically bent to form amicroscopic ring having a diameter less than about 15 micrometers.Further, FIG. 10 shows two 330-nm diameter wires 44 and 46 that aretwisted together. As shown in these figures, the silica wires do notbreak when bent and/or twisted, indicating that they have excellentflexibility and mechanical properties. In particular, a tensile strengthof at least 2.5 GPa for the silica wire shown in FIG. 10 can beestimated by employing the Young's modulus of silica (73.1 GPa). Silicawires of the invention can be bent even more sharply than shown above.For example, FIG. 11 shows a 280-nm diameter silica wire 48 bent to aradius of about 2.7 micrometers, indicating that the wire has a tensilestrength exceeding about 4.5 GPa. In fact, it has been found that thetensile strengths of prototype silica wires prepared in accordance withthe teachings of the invention are typically higher than about 5.5 GPa.Further, bending radii of less than 1 micrometer upon plastic bending ofsome silica wires of the invention can be achieved.

Sub-micrometer- or nanometer diameter silica wires of the invention finda variety of applications. For example, such wires can be utilized fortransmission of electromagnetic radiation between two spatial locations.In other words, such silica wires can be employed as optical fibers fortransmission of electromagnetic radiation. As discussed in more detailbelow, light can be coupled from an external source, e.g., anotherfiber, into a SMNW of the invention by employing evanescent wavecoupling. For example, FIG. 12 schematically illustrates an exemplaryexperimental set-up for launching light from one SMNW 50 onto anotherSMNW 52 that can guide the coupled light to a desired location. Moreparticularly, the SMNW 52 is suspended in air with one end fixed to asupport 54, and the other end connected to a fiber taper 56 from whichit was drawn. The launching wire 50, which is drawn from a second fibertaper 58, attaches itself to the guiding wire as a result of a van derWaals interaction between the wires. To reduce the interferingcontribution from the light due to scattering, both fiber tapers weregold-coated except for the region utilized for evanescent wave coupling.The coupling of radiation from an external source (not shown) into thelaunching fiber can be readily achieved via the second fiber taper 58that exhibits micrometer-sized cross-sectional diameter.

FIG. 13 shows an optical micrograph of coupling of light having awavelength of 633 nm from a 390-nm diameter silica launching wire onto a450-nm diameter silica wire, achieved by utilizing the aboveexperimental set-up. FIG. 14 shows the interception of light guided bythe 360-nm diameter wire (from left to right) by a supporting3-micrometer wire (shown at the right of the image extendingtransversely to the guiding wire), thereby illustrating that the amountof light scattered by the wire is small compared to that guided by it.To further illustrate light propagation properties of SMNW's formedaccording to the teachings of the invention, FIG. 15 shows a 550-nmdiameter wire guiding 633-nm light in air (on the left) and along asurface of a MgF₂ crystal (on the right). Because the refractive indexof MgF₂ is lower than that of silica, the silica wire continues to guidethe light on the MgF₂ surface, demonstrating the possibility ofintegrating SMNW's of the invention with low-index substrates for deviceapplications. In some preferred embodiments, the silica nanowire can bemounted on a silica aerogel substrate, as discussed in more detailbelow.

In other applications, nano-sized silica wires of the invention can beemployed for chemical and biosensing in liquid media. It has beendemonstrated that a 620-nm diameter silica wire formed according to theteachings of the invention can guide light, e.g., light having awavelength of 633 nm, in water micro-drops, thereby demonstrating thefeasibility of utilizing the silica wires for such applications.

FIG. 16 depicts two graphs illustrating experimentally obtained data foroptical loss of a number of SMNW's of the invention as a function ofwire diameter for radiation at two wavelengths of 633 nm and 1550 nm.The illustrated optical loss data was obtained by measuring opticaltransmission as a function of length L along the wire, e.g., between thecoupling region and fiber taper 1 in experimental set-up shown in FIG.12. Care was taken to maintain the same coupling efficiency between thelaunching and guiding wires. For example, when utilizing theexperimental set-up of FIG. 12, the overlap between the launching wireand the guide wire, whose optical loss was measured, was adjusted untilthe light output from the fiber taper 56 was maximized.

With continued reference to the graphs of FIG. 16, the measured opticalloss of a 190-nm diameter wire at a wavelength of 633 nm is about 1.7dB/mm, which is much lower than the optical loss of many conventionalsub-wavelength structures, such as metallic plasmon waveguides. Withoutbeing limited to any particular theory, the increasing optical loss withdecreasing wire diameter, shown in the graphs, can be attributed tosurface contamination. In particular, as the wire diameter is reducedbelow the wavelength of propagating light, more light is guided outsidethe wire's core as an evanescent wave, and is therefore more susceptibleto scattering by surface contamination. In particular, calculations showthat about 20% of energy propagates outside the silica core of a silicawire having a diameter of 450 nm at 633-nm wavelength, and outside thecore of a silica wire having a diameter of 1100 nm at 1550-nmwavelength. For smaller diameters, a larger fraction of the energytransmitted by the wire propagates outside the core as an evanescentwave.

Evanescent wave propagation exhibited by nano-meter sized silica wiresof the invention advantageously provides the possibility of effectivelyutilizing these wires in a variety of devices, such as optical sensors.Further, the high smoothness of the surface of a silica wire of theinvention lowers scattering of such an evanescent wave as it propagatesoutside the wire core in proximity of the wire's outer surface, therebyreducing optical loss of transmission.

As discussed above, the SMNW's formed according to the teachings of theinvention can be bent sharply without suffering fracture. In addition,light transmitted by the fibers can follow such sharp bends. In fact,because of a large index contrast between silica and air, silica SMNW'scan be bent sharply without incurring large bending losses fortransmission of optical energy. For example, 3-dimensional finitedifference time domain simulations can be utilized to find that anair-cladded 450-nm diameter silica wire shows a bending loss of lessthan about 0.3 dB for a 90 degree turn with a bending radius of 5micrometers for light at a wavelength of 633 nm.

As shown in FIG. 17, 633-nm wavelength light can be successfully guidedthrough a bend having a 5.6 micrometer radius in a silica wire 60 havinga diameter of about 510 nm. The observed intensity of the scatteredlight after the bend was not greatly reduced, thereby indicating a lowbending loss.

Thus, the nano-sized silica wires of the invention are particularlysuitable for use in applications in which tight waveguide bends aredesired. For example, the ability to guide light through sharp bends canbe especially useful for miniaturization of photonic devices. Microringsmade for such wires can be incorporated into photonic devices, such asoptical microresonators for optical communication or optical sensing. Toillustrate the feasibility of utilizing such wires in photonic devices,a 950-nm diameter SMNW formed in accordance with the teachings of theinvention was employed to form a ring with a 75-micrometer radius thatexhibited a Q-factor of about 1500 at a wavelength of 1550 mm.

The nano-sized silica wires according to the teachings of the inventioncan be employed in a variety of microphotonic devices, such as linearwaveguides, waveguide bends and branch couplers, as discussed in moredetail below. In many of these applications, one or more nano-sizedsilica wires are mounted on a substrate having an index of refractionthat is lower than that of the silica wires (preferably much lower than1.45). By way of example, as shown schematically in FIG. 18, in oneexemplary embodiment, a nano-sized silica wire 60 a having an averagediameter in a range of about 20 nm to about 1000 nm, and more preferablyin a range of about 20 nm to about 200 nm, is mounted on a substrate 62having an index of refraction less than about 1.4, e.g., in a rangeabout 1 to about 1.1, and more preferably in a range of about 1 to about1.05 (e.g., 1.03). In one preferred embodiment, the substrate 62supporting the nano-sized silica wire 60 is formed of silica aerogel,which is a silicon-based solid with a porous, sponge-like structure.More specifically, silica aerogel can comprise a network of silicananoparticles about 30 nm in size, much smaller than typical wavelengthsof guided light, and can exhibit a transparent optical spectral rangesimilar to that of silica. Silica aerogel, in which about 99 percent ofvolume can be air, is much less dense than glass (e.g., about 1000 timesless dense), and has an index of refraction very close to that of air(e.g., an index of refraction of about 1.03). Because the differencebetween the index of refraction of aerogel and air (a difference ofabout 0.03) is much less than a difference between the index ofrefraction of silica SMNW and air (a difference of about 0.45), theoptical guiding properties of silica aerogel-supported nano-meter sizedsilica wires are virtually identical to those of air-clad wires.

Although silica aerogel is identified herein as one example of suitablesubstrate, the substrate can be formed of aerogels of other materials,such as, alumina, tungsten oxide, ferric oxide and tin oxide.

In some embodiments of the invention, the assembly of silica nanowireson a solid substrate and their manipulation (e.g., cutting them into adesired length) can be achieved by employing scanning tunnelingmicroscope (STM) probes or other suitable microprobes. For example,using STM probes, silica SMNW can be cut, positioned, bent and twistedwith high precision under an optical microscope. By way of example, insome embodiments of the invention, a bend-to-fracture method can beapplied to cut a SMNW to a desired length by holding the wire with twoSTM probes on a substrate (e.g., silicon or sapphire substrate) andusing a third probe to bend the wire to cause fracture at a desiredpoint. This process leaves flat end faces at the fracture point even fora silica wire segment that is only about 1 micron in length, as shown inFIG. 19. When silica nanowires are placed on a substrate, e.g., a silicaaerogel substrate, they are held tightly in place by the van der Waalsattraction forces between the wires and the substrate. A silica nanowirecan also be repositioned or elastically bent to a desired radius byusing STM probes. By way of example, FIG. 20 shows a 200-nm diameterwire bent into a 9 micron diameter loop on a sapphire wafer. The wiremaintains its bent shape after removing the STM probes as a result offriction between the wire and the substrate.

In some embodiments, to avoid long-term fatigue and fracture of anelastically bent wire due to bending stress, a bent wire can be annealedat an elevated temperature for a selected duration to cause a permanentplastic deformation of the wire without degrading surface smoothness ordiameter uniformity. For example, the bent wire can be annealed at atemperature of about 1400 K in vacuum (e.g., 2×10⁻³ Pa) for about twohours. Generally, such annealing does not affect the geometry of one ormore nanowires assembled on a substrate because of the tight couplingbetween the wires and the substrate. This allows laying out a finaldesign geometry of wires on a substrate before performing the hightemperature annealing step. For example, FIG. 21 shows an SEM image of a5-micron radius plastic bend in a 410-nm silica wire mounted on asapphire substrate. The difference between the radii of the bend beforeand after annealing is less than about 0.5%, thus indicating that theannealing step does not significantly affect the pre-anneal geometry.The above process of bending a wire followed by annealing can beiterated to obtain very tight bends or multiple bends in nearbylocations, as shown in exemplary SEM images presented in FIGS. 22 and 23corresponding, respectively, to double plastic bends in a 940-nmdiameter silica wire and double bends in an 800-nm diameter silica wire(the sharp bend has a radius less than about 1 micron).

The optical properties of silica nanowires of the invention can betheoretically investigated by numerically solving the Maxwell equations.Such calculations indicate a single mode propagation through anano-sized silica wire having a normalized wire diameter

$\left( \frac{D}{\lambda_{0}} \right),$

defined as a ratio of the wire's physical diameter (D) to the wavelengthof propagating radiation (λ₀), that is less than about 0.73 (hereinreferred to as single-mode cut-off diameter). At the single-mode cut-offdiameter D_(SM) (e.g., 457 nm for propagating radiation having awavelength of 633 nm), more than about 80% of the light energy is guidedinside the wire. The remainder of the energy is guided as an evanescentwave. An exemplary distribution of the electric field intensity(calculated by employing a three-dimensional finite-differencetime-domain method) across the cross-section of a 450-nm diameter silicawire bent to a radius of about 5 microns shows that there is virtuallyno leakage of light through such a tight bend. Further, exemplarycalculations performed for coupling of light between two silica wires,each having a diameter of about 350 nm, that have an overlap of only 2.6microns along a longitudinal portions thereof indicate that more thanabout 97% of light originally propagating in one wire can be transferredto the other. In other words, the wire can provide a tight confinementof light, while the evanescent wave propagating outside the wire canfacilitate efficient coupling of light from one wire to another,launching of light into a wire, and building micrometer-sized opticalcouplers, as discussed in more detail below.

Silica nanowires according to the teachings of the invention can beutilized to transmit electromagnetic radiation having wavelengths over abroad spectral range as the ultraviolet (UV) and the infrared (IR)absorption edges of silica are, respectively, below 250 nm and beyond2000 nm. For example, such nano-sized silica wires can transmitelectromagnetic radiation with wavelengths in a range of about 250 toabout 2000 nm, e.g., in a range of about 400 nm to about 1600 nm.Similarly, a SMNW bend can be employed over a broad range of wavelengths(from the ultraviolet to the infrared). Hence, silica nanowires of theinvention provide much enhanced flexibility relative to photonic crystalstructures that can operate only at specific wavelengths.

As noted above, in many preferred embodiments of the invention, silicananowires can be mounted on a silica aerogel substrate. By way ofexample, FIG. 24 shows an SEM image of a 450-nm wide silica wire that issupported by a silica aerogel substrate while FIG. 25 shows an SEM imageof an arogel-supported 530-nm wide wire with a bending radius of 8microns. A silica nanowire supported by a silica areogel substrate canefficiently transmit electromagnetic radiation. For example, FIG. 26shows an optical microscopy image of a 380-nm diameter silica wireguiding radiation with a wavelength of 633 nm on a surface of a silicaaerogel. As the propagating light moves beyond the end face of the wire,it spreads out and scatters on the aerogel surface. The uniform andsubstantially unattenuated propagation along the nearly 0.5 millimeterslength of the wire and the strong output at the end face show lowscattering relative to guided intensity.

FIG. 27 presents optical loss data at a wavelength of 633 mm forexemplary illustrative straight silica wires supported by a silicaaerogel substrate, indicating a low loss even at wire diameters lessthan 500 mm. In fact, for wires having a diameter close to D_(SM), themeasured optical loss is less than about 0.06 db/m. This measured lowoptical loss corroborates that the silica aerogel does not degrade theproperties of the silica nanowires for guiding light, and opens the wayof fabricating microphotonic devices based on the silica nanowires.

In addition to straight silica nanowires, bent wires supported on asilica gel can also be employed for efficient transmission ofelectromagnetic radiation. FIG. 28 shows an optical microscopy image ofthe 530-nm diameter silica wire supported on an aerogel silicasubstrate, as described above in connection with FIG. 25, guiding lighthaving a wavelength of 633 nm around a bend with a radius of 8 microns.FIG. 29 presents data corresponding to measured optical loss as afunction of bending radius in a 530 nm diameter silica wire, indicatingthat the losses are sufficiently small to be acceptable forincorporation of such bent wires in photonic devices. For example, themeasured optical loss around a 5 micron radius bend is less than 1 dB.Hence, nanosized silica wires provide the advantages of compact overallsize, low coupling loss, simple structure and easy fabrication. Incontrast, bending waveguides based on planar photonic crystal structuresnot only require multiple periods, with concomitant increase in sizes ofthe structures, and complex fabrication techniques, but they alsotypically exhibit out-of-plane optical loss.

As noted above, silica nanowires according to the teachings of theinvention can be employed as building blocks for fabricatingmicrophotonic devices on a suitable substrate, e.g., a silica aerogelsubstrate. For example, FIG. 30 schematically depicts a microphotonicradiation coupler 64 comprising two straight silica nanowires 66 and 68mounted on a silica aerogel substrate 70. The nanowire 66 extends from aproximal end 66 a to a distal end 66 b and the nanowire 68 extends froma proximal end 68 a to a distal end 68 b. The nanowires are disposed onthe substrate 64 such that a distal portion 66 c of the nanowire 66 hasa length-wise contact (optical coupling) with a proximal portion 68 c ofthe nanowire 68 over a distance D that is selected to as to causecoupling of radiation originally guided along one wire into the otherwire. For example, radiation originally coupled via the proximal portionof the nanowire 66 and guided along this wire along a direction A(schematically depicted by an arrow) can be coupled into the proximalportion of the wire 68 to be guided towards its distal end 68 b along adirection schematically designated by an arrow B. The overlap distance Dcan be in a range of about 1 micrometer to about 100 micrometers (e.g.,in a range of about 2 to about 4 micrometers), or any other lengthsuitable for providing an efficient coupling between the two wires.

In another application, the silica nanowires according to the teachingsof the invention can be utilized to fabricate microphotonic X-couplers.For example, FIG. 31 schematically depicts an X-coupler 72 formed of twobent silica nanowires 74 and 76 assembled on a silica aerogel substrate78 so as to have an overlap (contact) along a portion 80, herein alsoreferred to as an interaction region. Electromagnetic radiationoriginally launched into the nanowire 76 (or alternatively along thenanowire 74) along an arrow A is split by the optical coupling betweenthe wires in the interaction region into two radiations flow paths, oneprimarily guided by the wire 74 along an arrow A, and the other guidedby the wire 76 along an arrow C. In other words, the interaction regioncauses coupling of a fraction of radiation originally guided by one ofthe wires to the other wire while the remaining radiation continues itspropagation along the wire initially guiding the radiation. Theinteraction region can have a length in a range of about 1 micrometer toabout 100 micrometers (e.g., in a range of about 2 to about 4 microns).To illustrate the feasibility of forming such microphotonic X-couplers,a prototype X-coupler was formed of by two 420-nm diameter silica wirebends, assembled on a silica aerogel substrate, with an overlap of about5 microns. The prototype X-coupler functioned as a 3-dB radiationsplitter with an excess loss of less than 0.5 dB. By changing theoverlap between the two bends over a range of about 3 microns, it ispossible to tune the splitting ratio of such a coupler (a fraction ofradiation coupled from one wire to the other) from less than 5% to morethan 90%. Hence, the silica nanowires of the invention can be utilizedto develop tunable microphotonic devices, such as tunable couplers andswitches.

Couplers assembled with SMNWs according to the teachings of theinvention provide a number of advantages over conventional microscopiccouplers, such as fused couplers made from fiber tapers usingconventional methods that typically require a much longer interactionlength (e.g., of the order of 100 microns). In comparison with suchconventional couplers, utilizing SMNWs to assembler coupler can resultin reducing coupler size by more than an order of magnitude.

In some microphotonic devices, two silica nanowires mounted on a silicaaerogel substrate, or any other suitable substrate, can intersect withone another, e.g., at a right angle (90 degree angle), as shownschematically in FIG. 32 (showing nanowires 82, 84 assembled on anaerogel substrate 86), with minimal cross-talk between the wires. Forexample, FIG. 33 is an optical microscope image of two 390-nm diametersilica nanowires supported by a silica aerogel substrate that intersectperpendicularly, into one of which (the horizontal wire) light having awavelength of 633-nm is launched. Except for some weak scattering at theintersection of the wires, virtually no light is coupled from thehorizontal wire to the vertical one. A measured cross-talk between thetwo wires is about −35 dB. The intentionally induced strong scatteringon the right side of the horizontal wire provides a qualitativeindication of the relative intensity of the guided light. Intersectingsilica wires with larger diameters can exhibit even better isolation.Hence, although light can be readily coupled between two silicananowires, e.g., by providing a longitudinal overlap between the wiresas discussed above, vertically intersecting wires do not cause an“optical shortcut.” This lack of cross-talk between verticallyintersecting wires is particularly advantageous for high-density opticalintegration.

With reference to FIG. 34, in some embodiments of the invention, anano-sized silica fiber 11 having cross-sectional diameters in a rangeof about 20 nm to about 1000 nm can be doped with a selected dopant 13.Such doped fibers can be employed in a variety active devices. Forexample, the dopants can be chosen to provide, for example,photoluminescence or lasing centers. Alternatively, the doped nano-sizedsilica fibers can be utilized to fabricate wavelength multiplexers. Byway of example, in some embodiments, the dopants can comprise Erbiumions (e.g., Er⁺³). Other possible dopants include, without limitation,Nd⁺³ and Yb⁺³. In one method of forming such doped nanowires, dopedsilica fibers having larger diameters (e.g., micron-sized diameters) canbe drawn by utilizing the methods described herein to form nanowireshaving diameters in a range of about 20 nm to about 1000 nm.

A two-step process for fabricating silica nanowires having nano-metersized diameters was discussed above. In another embodiment, the silicananowires are formed by employing a self-modulated taper-drawingprocess, discussed in more detail below. FIG. 35 schematicallyillustrates a self-modulated taper-drawing system 88 according to thisembodiment of the invention that is suitable for generating nano-sizedsilica wires. The System 88 includes a tapered sapphire rod 90 having atip portion 90 a. Initially, a standard single-mode silica fiber 92 isheated and drawn to cause a portion thereof to form amicrometer-diameter wire coupled to the remainder of the fiber throughtapered segment 92 a. A portion of the micrometer-diameter wire iswrapped about the tip of the sapphire rod, and the silica fiber (towhich the wire is connected through a taper) is made substantiallyparallel to the sapphire rod without breaking the connection between thesilica wire and the sapphire taper. A 90 degree bend is then made closeto the tapered segment, e.g., by tightening the wire, so as to generatea tensile force along the wire, which can be used for self modulation.In this exemplary embodiment, a three-dimensional stage 94, to which anend of the standard fiber close to the tapered segment is anchored, canbe employed to facilitate formation of the bend and the subsequentdrawing step. The exemplary system 88 also includes a 30-mWcontinuous-wave He—Ne laser 96 (633-nm wavelength) to launch light, viaan objective lens 98, into the silica fiber, the fiber taper, themicro-sized fiber and the wire being drawn so as to facilitate real-timemonitoring and controlling of the drawing process.

Since the sapphire taper is much thicker and stiffer that the silicafiber taper, the elastic bend occurs at the tapered segment. The weightof the vertical portion of the silica wire can cause the wire to bedrawn as the tip portion of the sapphire rod is heated (e.g., via aflame applied to a portion of rod a selected distance from the tip) tocause softening of the wire wrapped around the tip. The temperature ofthe tip portion can be in a range of about 1500 C to about 1800 C (e.g.,1600 C). Initially, the drawing force can be relatively large as thewire has substantially its initial diameter. However, as the drawingcontinues, the diameter of the wire decreases and the fiber bend movesfrom the thicker end of the fiber taper to its thinner end, resulting ina smaller drawing force, as shown schematically in FIG. 36. In thismanner, the force is modulated to allow drawing wires having very smalland uniform diameters. In addition, in case of unpredictable changes indrawing conditions (for example, a slight temperature fluctuation thatmay cause a variation in viscosity of silica), the center of the elasticbend can shift to modulate the drawing speed to prevent a variation inwire diameter or its breakage. A drawn wire can include three parts: afirst tapered end (typically millimeters long), a uniform segment (up tocentimeters), and a second abruptly tapered end

The above self-modulated taper drawing technique was employed togenerate silica wires having diameters as small as about 20 nanometers.FIG. 37 presents graphs 100 and 102 corresponding, respectively, tomeasured diameter (D) and diameter uniformity (U_(D)) as a function oflength for a prototype nano-sized silica wire fabricated by employingthe above self-modulated taper drawing process. The diameter uniformity(U_(D)) is defined as

$\frac{\Delta \; D}{L},$

where L designates a length over which the wire exhibits a diameterdeviation (ΔD) from a central, or an average, diameter D. Although thewire exhibits a monotonously tapering tendency on the whole, its uniformportions shows a very high degree of diameter uniformity. For example,at D=30 nm, the diameter uniformity (U_(D)) is about 1.2×10⁻⁵. In otherwords, the maximum diameter difference between the two ends of a wire,having a length of 80 microns and a diameter centered around 30 microns,is less than about 1 micron. Even for a very thin wire having a diametercentered around 20 nm, the diameter uniformity (U_(D)) can be less than10⁻³. It should be understood that the above data are provided only forillustration purposes, and the fabrication methods of the invention canalso be employed to generate wires having smaller diameters and betterdiameter uniformity than those described above.

Silica nanowires formed according to the teachings of the invention alsoexhibit smooth sidewalls. For example, high resolution TEM was employedto measure a root-mean-square roughness of less than about 0.2 nm forthe sidewall of a prototype 35-nm diameter silica nanowire fabricated byutilizing the above self-modulated taper drawing process. Consideringthat the length of an Si—O bond is about 0.16 nm, such a low sidewallroughness indicates an atomic-level smoothness of the wire surface. Inaddition, an electron diffraction pattern of this nanowire indicates anamorphous composition.

In some applications, the silica nanowires of the invention can beemployed to form a nanowire bundle, e.g., by attaching them by utilizingvan der Waals attractive forces. By way of example, FIG. 38 shows an SEMimage of three silica nanowires 104 a (30-nm diameter), 104 b (140-nmdiameter) and 104 c (510-nm diameter) that are attached to one anotheralong their lengths via van der Waals attraction to form a nanowirebundle.

The microphotonic applications of silica nanowires of the invention arenot limited to those described above. As another example, FIG. 39 showsan SEM image of an 18 micron diameter loop assembled with a 65-nmdiameter silica nanowire, which can be used as anoring for optical andmechanical purposes. By way of other examples, FIG. 40 shows an SEMimage of a rope-like twist formed with a 120-nm diameter silica nanowireon a silicon substrate while FIG. 41 presents an SEM image of a 150-nmdiameter silica nanowire wrapped around a 4 micron diameter fiber toform a spiral coil, which can be incorporated in nanodevices, e.g., as ananospring.

Those having ordinary skill in the art will appreciate that variouschanges can be made to the above embodiments without departing from thescope of the invention. For example, the support element can be formedfrom materials other than those described above, and a variety of heatsources can be employed for heating the support element.

1-24. (canceled)
 25. A microphotonic device, comprising a substratehaving an index of refraction in a range of about 1 to about 1.4, one ormore silica nanowires having an average diameter in a range of about 20nm to about 1000 nm mounted on a surface of said substrate.
 26. Themicorphotonic device of claim 25, wherein said substrate has an index ofrefraction in a range of about 1 to about 1.05.
 27. The microphotonicdevice of claim 25, wherein said substrate had an index of refraction ina range of about 1 to about 1.03.
 28. The microphotonic device of claim25, wherein said substrate comprises silica aerogel.
 29. Themicrophotonic device of claim 28, wherein said one or more silica wireshave average diameters in a range of about 20 nm to about 500 nm. 30.The microphotonic device of claim 28, wherein said one or more silicawires have average diameters in a range of about 20 nm to about 200 nm.31. The microphotonic device of claim 28, wherein said one or moresilica wires have average diameters in a range of about 20 nm to about100 nm.
 32. The microphotonic device of claim 28, wherein said one ormore silica wires have average diameters in a range of about 20 nm toabout 50 nm.
 33. A microphotonic branch coupler, comprising a substratehaving an index of refraction in a range of about 1 to about 1.4, andtwo silica fibers having average diameters in a range of about 20 nm toabout 1000 nm mounted on said substrate to have longitudinal contactover a distance selected, said contact allowing a fraction of radiationoriginally guided by one fiber to couple to the other fiber while theremaining radiation continues its propagation in the originalradiation-guiding fiber.
 34. The microphotonic coupler of claim 33,wherein said coupling distance is in a range of about 1 micron to about100 microns.
 35. The microphotonic coupler of claim 33, wherein saidsubstrate has an index of refraction in a range of about 1 to about 1.1.36. The microphotonic coupler of claim 35, wherein said substratecomprises silica aerogel.
 37. The microphotonic coupler of claim 33,wherein at least one of said fibers is generally U-shaped.
 38. Amicrophotonic coupler, comprising a substrate having an index ofrefraction in a range of about 1 to about 1.4, and two silica fibersassembled on said substrate, each fiber having a proximal end and adistal end and an average diameter in a range of about 20 nm to about1000 nm, a distal portion of one fiber being in length-wise contact witha proximal portion of the other fiber such that radiation guided by onefiber can couple at said contact to the other fiber.
 39. Themicrophotonic coupler of claim 38, wherein said substrate has an indexof refraction in a range of about 1 to about 1.1.
 40. The microphotoniccoupler of claim 38, wherein said substrate comprises silica aerogel.41-43. (canceled)
 44. A fiber, comprising a silica nanowire having aconcentration of a dopant distributed therein, said nanowire having anaverage diameter in a range of about 20 nm to about 1000 nm.
 45. Thefiber of claim 44, wherein said nanowire has an average diameter in arange of about 20 nm to about 500 nm.
 46. The fiber of claim 44, whereinsaid nanowire has an average diameter in a range of about 20 nm to about100 nm.
 47. The fiber of claim 44, wherein said dopant can be any ofEr⁺³, Nd⁺³ or Yb⁺³.